Kanneliya-Dediyagala-Nakiyadeniya (KDN) Biosphere Reserve, which covers lowland and mountain tropical forests, occupies the interlude between two rivers. It consists of a series of parallel strikes - ridges and valleys. The area is covered by a natural high forest ecosystem consisting of several forest types, mainly lowland tropical forest, wet semi-evergreen forest, submontane tropical wet semi-evergreen forest and montane tropical wet-evergreen forest. It is the only relatively large extension of tropical rainforest of this type in Sri Lanka with 'Dipterocarps' as the dominant forest composition. In the KDN complex, a total of 319 woody plant species representing 194 genera have been identified, of which 22% are endangered, 27% vulnerable and 45% rare. Some 220 faunal species including 41 endemic species have been recorded such as the white-throated flowerpecker (Dicaeum vincens), Ceylon myna (Gracula ptilogenys) and Sri Lanka hanging parrot (Loriculus beryllinus).

The Biosphere Reserve is a major catchment area for two of the most important rivers in the region, the Gin and Nilwala Rivers, which feeds numerous rivers and streams. They drain into the Indian Ocean near the world famous coral reefs of the Hikkaduwa Marine Sanctuary. These two rivers are vital for the regional agricultural and industrial development in the Galle and Matara Districts.

10,000 people live in 78 villages in the transition area of the Biosphere Reserve. In the core area, only scientific research and ecotourism activities are permitted. The main economic activities take place in the buffer and transition areas and include forestry, collection of non-timber products, paddy fields, tea, rubber and cinnamon plantations, animal husbandry and other forms of agriculture and cottage industries. There are three temple sites in the KDN complex, the Nugegoda, Rajagala and Dediyagala. These are hermitage type temples or shrines within the forest.